There have been a number of previously known compound engines having two diverse engine types which are coupled together to provide a common output. Such an engine is described in U.S. Pat. No. 4,742,683 which couples a rotary Wankel engine with a turbine engine; U.S. Pat. No. 4,586,337 which couples a turbocharged reciprocating engine with a power turbine; U.S. Pat. Nos. 4,843,821, and 4,791,787 which couple a reciprocal engine with a rotary Wankel engine.
These compounded engines are two separate engines of different types used to generate power, and are to be distinguished from a turbocharged engine which has a small turbine with an impeller driven by the exhaust of an engine, and which drives a compressor to compress air to supercharge the engine. Various designs have been developed for alternate uses of the turbochargers, as illustrated by U.S. Pat. No. 4,449,370 which has a turbocharged diesel engine coupled with a turbocharger that may be independently driven by a catalytic combustor in order to generate electric power when the diesel is not running.
Compound engines enjoy many advantages in certain applications, and seek to take advantage of the optimum efficiencies of each particular engine type over the particular operating range most suitable for each particular engine type. For example, one of the diverse engine types is particularly efficient during certain operating conditions while, conversely, the other engine type enjoys certain efficiencies during different operating conditions. For this reason, in prior compounded systems one engine type complements the other engine type by providing a second, separately fueled engine, thus providing overall engine efficiencies over a wider range of operating conditions of the compound engine than is available from each engine independently.
For example, the engine described in U.S. Pat. No. 4,586,337 has a combustor in which fuel is injected and ignited in order to drive a turbine, with the exhaust from a compounded Wankel engine being fed into the combustor along with a variable amount of compressed air. The ratio of power output between the two engines may be varied, with one increasing while the other decreases, but operating over a broader range than available with either engine independently.
One difficulty with engines, which may become worse with compounded engines, is the generation of excess pollutants. If the exhaust of one engine is fed into the combustion chamber of another engine, the increased combustion temperature of the second engine in combination with the incoming fuel and oxygen may generate increased nitrogen oxide pollutants. If one of the compounded engines is operating inefficiently, then unburned hydrocarbon pollutants may be expelled in the exhaust. There is thus a need for a compounded system which reduces the amount of pollutants generated.
Many compound engines have been of the reciprocating piston type, and numerous devices have been devised which attempt to enhance the power produced from the exhaust. Disadvantageously, the turbocompounding of reciprocating internal combustion engines causes adverse back pressure into the piston combustion chamber, and a decreased pressure drop on any turbocharger used with the compounded engine. Both effects cause a reduction in power. There is thus a need for a turbocompounded engine which performs efficiently at higher backpressures.
No matter how efficient the engine, a large amount of waste energy is often expelled through the exhaust, or through the lubricants and coolants. Adiabatic turbocompounded diesel engines are being investigated which use expensive ceramic materials to allow higher temperature adiabatic combustion in order to increase the engine efficiency. Such engines, however, are expensive, and require exotic materials and parts which are neither widely nor commercially available.
Cogeneration systems have been developed to take advantage of the heat available in the exhaust, lubricants, or coolants of conventional internal combustion engines. Such systems typically use heat exchangers to recapture a portion of the heat from conventional internal combustion engines, with the engines being used to drive an electric generator. While such cogeneration systems use available components, they are bulky, heavy, of limited power, of limited generation capability, and sometimes present disadvantages in providing the liquid fuel in environments where a number of people are present.
There is thus a need for a compound engine which overcomes these disadvantages, but offers the advantages in weight and size as other turbocompounded engines, and has the added advantages of extreme fuel efficiency and reduction of exhaust pollutants. There is a further need for using such a compounded engine in a cogeneration application to derive the maximum energy possible from the system while generating the minimum pollutants.